Aqueous enamels contain resins which form stable solutions or dispersions through ionic (anionic or cationic) or non-ionic stabilization in water. These resins which do not dry purely physically (i.e., without addition of a hardener after removal of the solvent or dispersant, they form a film which is only cross-linked through physical interactions such as dispersion or van der Waals interactions or through hydrogen bridge-type bonds), but rather dry chemically, i.e., through reaction with a hardener or cross-linking agent, have further reactive groups which cross-link with the hardener to form covalent bonds during the curing procedure. With stoving enamels, this cross-linking only sets on at higher temperatures; therefore, in this case mixtures of resin and hardener may be manufactured and stored at room temperature for a long time before the processing. The hardener must either become reactive only at higher temperatures or be sufficiently deactivated by protective groups that no premature cross-linking reaction occurs during storage.
Lowering the cross-linking temperature for these stoving enamels (and thereby saving time and/or energy during the thermal cross-linking), without, however, negatively influencing the storability, is desirable.
If an etherified aminoplast hardener is used for the cross-linking, which reacts with resins containing hydroxyl groups with transetherification and formation of N-methylene ether bonds, the presence of acids as catalysts is necessary. The transetherification only occurs under acid catalysis. For water-dilutable anionically stabilized resins, which are hydrophilically modified by incorporating acid groups and at least partially neutralizing these acid groups, amines are typically used for neutralization. These amines must have, to a large extent, left the enamel film to be cross-linked so that the acids used as the catalyst may become active. At lower temperatures (below 100° C.) the volatility of the amines used for naturalization is still low, however, and therefore the hardening reaction may not yet begin at these temperatures.